The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle. More particularly, the present invention is directed to a transfer clutch adapted for use in motor vehicle driveline applications having a magnetorheological clutch actuator that is operable for controlling actuation of a ball-screw operator associated with a multi-plate clutch assembly.
In view of increased demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In many vehicles, a transfer case is interconnected between the primary and secondary drivelines which is equipped with a dog-type mode clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a part-time four-wheel drive mode. In contrast, the mode clutch is disengaged, drive torque is only delivered to the primary driveline for establishing a two-wheel drive mode.
A modern trend in four-wheel drive motor vehicles is to equip the transfer case with an electronically-controlled transfer clutch in place of the mode clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an on-demand four-wheel drive mode. Typically, the transfer clutch includes a multiplate clutch assembly that is installed between the primary and secondary output shafts of the transfer case and which is actuated by a power-operated actuator in response to control signals sent from a controller. The control signals are typically based on current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such xe2x80x9con-demandxe2x80x9d transfer cases can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions.
In many instances, the vehicle operator is also permitted to select between the two-wheel drive mode and the part-time four-wheel drive mode in addition to the on-demand four-wheel drive mode. Specifically, when the two-wheel drive mode is selected, the clutch assembly is released for delivering all drive torque to the primary output shaft. In contrast, when the part-time four-wheel drive mode is selected, the clutch assembly is fully engaged for effectively locking the secondary output shaft to the primary output shaft. In such applications, a mode signal is sent to the controller which is indicative of the particular drive mode selected by the vehicle operator. Typically, the mode signal is generated by a mode selector device which is manipulated by the vehicle operator.
Currently, a large number of on-demand transfer cases are equipped with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary output shaft as a function of the value of an electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated actuator. For example, U.S. Pat. No. 5,407,024 discloses a electromagnetic coil that is incrementally activated to control movement of a ball-ramp operator for applying a clutch engagement force on a multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly.
As an alternative, the transfer clutch can employ an electric motor and a drive assembly as the power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying a variable clutch engagement force on a multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging a multi-plate clutch assembly. Finally, U.S. Pat. No. 4,895,236 discloses a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator which, in turn, applies the clutch engagement force to the clutch pack.
While many on-demand clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, the size, weight and electrical power requirements of the electromagnetic coil or the electric motors needed to provide the described clutch engagement loads may make such system cost prohibitive in some four-wheel drive vehicle applications. In an effort to address these concerns, new technologies are being considered for use in power-operated clutch actuator applications such as, for example, magnetorheological clutch actuators. One example of such an arrangement is described in U.S. Pat. No. 5,915,513 wherein a magnetorheological actuator controls operation of a ball-ramp unit to engage the clutch pack. While such an arrangement may appear to advance the art, its complexity clearly illustrates the need to continue development of even further defined advancement.
Thus, its is an object of the present invention to provide a transfer clutch having a magnetorheological clutch actuator that is operable for controlling movement of a thrust-generating clutch operator used to engage a multi-plate clutch assembly.
It is a further object of the present invention to provide a double-acting self-centering ball screw clutch operator in conjunction with a magnetorheological clutch actuator for use in a transfer clutch.
As a related object, the transfer clutch of the present invention is well-suited for use in motor vehicle driveline applications to control the transfer of drive torque between an input member and an output member.
According to a preferred embodiment, the transfer clutch includes a magnetorheological actuator which controls operation of a ball screw operator for controlling the magnitude of clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between an input member and an output member. The magnetorheological actuator includes an electromagnetic coil, and a rotor having a first segment retained in a sealed chamber filled with magnetorheological fluid. The ball screw operator includes a threaded screw mounted on the output member and which is splined to a second segment of the rotor, a threaded nut, and a plurality of balls retained between the aligned threads of the screw and nut. In addition, a drag spring provides a predetermined drag force between the screw and the output member. The multi-plate clutch assembly includes a drum driven by the input member, a hub driving the output member, and a clutch pack operably disposed between the drum and hub. The clutch assembly also includes a first pressure plate adapted to act on one end of the clutch pack and which couples the nut for rotation with the hub, a second pressure plate adapted to act on the other end of the clutch pack and which is driven by the drum, and a return spring acting between the first and second pressure plates. A housing is driven by the drum and defines the fluid chamber within which the first segment of the rotor is retained.
In operation, activation of the electromagnetic coil creates a magnetic flux field which travels through the housing and into the magnetorheological fluid for changing its viscosity and creating drag between the housing and the rotor, thereby causing relative rotation between the screw and nut of the ball screw operator. As such, relative rotation in a first direction causes axial movement of the threaded nut in a first direction which, in turn, causes the first pressure plate to exert a clutch engagement force on the clutch pack. Likewise, relative rotation between the screw and nut in the opposite direction causes axial movement of the nut in a second direction which, in turn, causes the second pressure plate to engage the clutch pack. Upon deactivation of the coil, the return spring forcibly separates the pressure plates to release the clutch pack and also acts to automatically center the nut relative to the screw.